Potential of hexavalent chromium-resistant rhizosphere bacteria in promoting plant growth and hexavalent chromium reduction

Aim: This research aimed to evaluate three Cr(VI)-resistant rhizosphere bacteria (Bacillus cereus, Bacillus aeries and Exiguoloacterium profundum) for their ability to produce plant growth-promoting (PGP) substances and to remove Cr(V1). Methodology: Three rhizosphere bacteria were characterized for their ability to produce several PGP substances, including ammonia and indole-3-acetic acid (IAA) and solubilized phosphate. The Cr(VI) reduction ability of the rhizosphere bacteria was determined by diphenylcarbazide method with 60 mg L-1 C-r(V1). Results: Bioreduction of Cr(VI) in the reduction media by B. cereus was 13.7%, which was higher compared with those of B. aerius (4.4%) and E. profundurn (3.6%). B. aeries adsorbed high Cr(VI) concentration of 56.1 mg g1. The best condition for these rhizobacteria to remove or adsorb Cr(VI) was at acidic pH (5.36-5.97). All rhizobacteria could not stand the toxic effect of Cr(VI) at 60 mg 11, which decreased almost 100% of rhizobacteria growth. B. cereus and B. aerius produced PGP substances, including ammonia and 1AAand solubilized phosphate. Interpretation: B. cereus and B. aerius with high PGP activities can be considered promising agents in microbe-assisted phytoremediation. All rhizosphere bacteria tolerated Cr(V1) and protected plants against the inhibitory effect of Cr(VI) by reducing Cr(VI) to Cr(III) and assisting the plant uptake of Cr(III).